Quantification of nitrite and nitrate in seawater by triethyloxonium tetrafluoroborate derivatization-headspace SPME GC-MS

Triethyloxonium tetrafluoroborate derivatization combined with direct headspace (HS) or SPME-gas chromatography-mass spectrometry (GC-MS) is proposed here for the simultaneous determination of nitrite and nitrate in seawater at micromolar level after conversion to their corresponding volatile ethyl-esters (EtO-NO and EtO-NO₂). Isotopically enriched nitrite [¹⁵N] and nitrate [¹⁵N] are employed as internal standards and for quantification purposes. HS-GC-MS provided instrumental detection limits of 0.07 μM NO₂⁻ and 2 μM NO₃⁻. Validation of the methodology was achieved by determination of nitrite and nitrate in MOOS-1 (Seawater Certified Reference Material for Nutrients, NRC Canada), yielding results in excellent agreement with certified values. All critical aspects connected with the potential inter-conversion between nitrite and nitrate (less than 10%) were evaluated and corrected for by the use of the isotopically enriched internal standard.     

Determination of Nitric Oxide-Derived Nitrite and Nitrate in Biological Samples by HPLC Coupled to Nitrite Oxidation

Nitrite and nitrate are main stable products of nitric oxide, a pivotal cellular signaling molecule, in biological fluids. Therefore, accurate measurement of the two ions is profoundly important. Nitrite is difficult to be determined for a larger number of interferences and unstable in the presence of oxygen. In this paper, a simple, cost-effective and accurate HPLC method for the determination of nitrite and nitrate was developed. On the basis of the reaction that nitrite is oxidized rapidly to nitrate with the addition of acidic potassium permanganate, the determination of nitrite and nitrate was achieved by the following strategy: each sample was injected twice for HPLC analysis, i.e. the first injection was to measure nitrate, and the second injection was to measure total nitrate including initial nitrate and the nitrate from the conversion of nitrite with the addition of acid potassium permanganate in the sample. The amount of nitrite can be calculated as difference between injections 2 and 1. The HPLC separation was performed on a reversed phase C₁₈ column for 15 min. The mobile phase consisted of methanol–water (2:98 by volume); the water in the mobile phase contained 0.60 mM phosphate salt (potassium dihydrogen and disodium hydrogen phosphate) and 2.5 mM tetrabutylammonium perchlorate (TBAP). The UV wavelength was set at 210 nm. Additionally, we systemically investigated the effects of the concentration of phosphate salt and TBAP in the mobile phase, the pH of the mobile phase, and the amount of acidic potassium permanganate added to the sample on the separation efficacy. The results showed that the limits of detection (LOD) and the limit of quantitation (LOQ) were 0.075 and 0.25 μM for nitrate (containing the oxidized nitrite), respectively. The linear range was 1–800 μM. This developed approach was successfully applied to assay nitrite/nitrate levels in cell culture medium, cell lysate, rat plasma and urine.

     

Determination of Nitric Oxide-Derived Nitrite and Nitrate in Biological Samples by HPLC Coupled to Nitrite Oxidation

Nitrite and nitrate are main stable products of nitric oxide, a pivotal cellular signaling molecule, in biological fluids. Therefore, accurate measurement of the two ions is profoundly important. Nitrite is difficult to be determined for a larger number of interferences and unstable in the presence of oxygen. In this paper, a simple, cost-effective and accurate HPLC method for the determination of nitrite and nitrate was developed. On the basis of the reaction that nitrite is oxidized rapidly to nitrate with the addition of acidic potassium permanganate, the determination of nitrite and nitrate was achieved by the following strategy: each sample was injected twice for HPLC analysis, i.e. the first injection was to measure nitrate, and the second injection was to measure total nitrate including initial nitrate and the nitrate from the conversion of nitrite with the addition of acid potassium permanganate in the sample. The amount of nitrite can be calculated as difference between injections 2 and 1. The HPLC separation was performed on a reversed phase C₁₈ column for 15 min. The mobile phase consisted of methanol–water (2:98 by volume); the water in the mobile phase contained 0.60 mM phosphate salt (potassium dihydrogen and disodium hydrogen phosphate) and 2.5 mM tetrabutylammonium perchlorate (TBAP). The UV wavelength was set at 210 nm. Additionally, we systemically investigated the effects of the concentration of phosphate salt and TBAP in the mobile phase, the pH of the mobile phase, and the amount of acidic potassium permanganate added to the sample on the separation efficacy. The results showed that the limits of detection (LOD) and the limit of quantitation (LOQ) were 0.075 and 0.25 μM for nitrate (containing the oxidized nitrite), respectively. The linear range was 1–800 μM. This developed approach was successfully applied to assay nitrite/nitrate levels in cell culture medium, cell lysate, rat plasma and urine.     

Amperometric detection of nitrite and nitrate at tetraruthenated porphyrin-modified electrodes in a continuous-flow assembly

The modification of a glassy carbon surface by coating with an electrostatically assembled film of tetraruthenated cobalt porphyrin/(meso-tetra(4-sulphonatephenyl)porphyrinate zinc(II) yields an indicator electrode that allows the determination of nitrite to be performed with a limit of detection of 0.1 μM in a flow injection configuration. The dynamic range extends up to 1000 μM and the repeatability of the measurements was evaluated to be 1.5% with a throughput of 50 samples per hour. The efficiency of the bilayered film to mediate the electron transfer allows the determinations to be performed at a less positive potential (+0.75 V) with enhanced sensitivity. The coating also prevents the surface poisoning and its stability is maintained over several weeks. The same detector was used for determination of nitrate after reduction to nitrite in a reductor column containing copperised cadmium. This method was used for the determination of nitrate and nitrite in mineral water, saliva and cured meats, the results being in agreement with certified values and those obtained by using recommended procedures.     

Simultaneous electrochemical determination of nitrate and nitrite in aqueous solution using Ag-doped zeolite-expanded graphite-epoxy electrode

In this work a new electrochemical sensor based on an Ag-doped zeolite-expanded graphite-epoxy composite electrode (AgZEGE) was evaluated as a novel alternative for the simultaneous quantitative determination of nitrate and nitrite in aqueous solutions. Cyclic voltammetry was used to characterize the electrochemical behavior of the electrode in the presence of individual or mixtures of nitrate and nitrite anions in 0.1 M Na₂SO₄ supporting electrolyte. Linear dependences of current versus nitrate and nitrite concentrations were obtained for the concentration ranges of 1-10 mM for nitrate and 0.1-1 mM for nitrite using cyclic voltammetry (CV), chronoamperometry (CA), and multiple-pulsed amperometry (MPA) procedures. The comparative assessment of the electrochemical behavior of the individual anions and mixtures of anions on this modified electrode allowed determining the working conditions for the simultaneous detection of the nitrite and nitrate anions. Applying MPA allowed enhancement of the sensitivity for direct and indirect nitrate detection and also for nitrite detection. The proposed sensor was applied in tap water samples spiked with known nitrate and nitrite concentrations and the results were in agreement with those obtained by a comparative spectrophotometric method. This work demonstrates that using multiple-pulse amperometry with the Ag-doped zeolite-expanded graphite-epoxy composite electrode provides a real opportunity for the simultaneous detection of nitrite and nitrate in aqueous solutions.     

基于量子点增敏化学发光同时测定尿液中的亚硝酸盐和硝酸盐

采用铜镉柱还原硝酸盐,与CdTe量子点增敏过氧亚硝酸-碳酸钠体系的化学发光信号相结合,开发了快速在线同时分析亚硝酸盐和硝酸盐的新方法.对流动注射、化学发光等实验参数条件进行优化,在Na2CO3的浓度为0.2 M、H2O2的浓度为0.03 M、Na2EDTA的浓度为1×10-3 M、CdTe量子点粒径为2.84 nm的条件下,过氧亚硝酸-碳酸钠体系可以获得最优的化学发光信号.该方法检测亚硝酸盐的线性范围为0.3~75μM,检测限可达0.12μM,其相对标准偏差为1.9%;硝酸盐的线性范围为1.0~100μM,检测限可达0.26μM,其相对标准偏差为1.5%.此方法无需衍生和分离,可以实现同时、准确、快速和高选择性地检测人体尿液中亚硝酸盐和硝酸盐的含量,回收率分别为94%~105%和96.6%~110.4%.     

Kinetic determination of uranium(IV) based on its catalytic effect on the phosphomolybdic acid-iodide reaction

A kinetic spectrophotometric method has been developed for the determination of trace amounts of uranium(IV) based on its catalytic effect on the phosphomolybdic acid iodide reaction. A significant feature of the proposed procedure is the selectivity for uranium(IV); it enables the determination of trace amount of uranium in the presence of large excess of rare metalions and other cations and anions. The method can be applied to the determination of uranium within the concentration range of 0–12μg· cm^?3, and the detection limit of the method is 0.02μg·cm^?3. Trace amounts of uranium in thorium nitrate and scandium oxide had been determined by the procedure and the results are satisfactory.     

A rapid technique for determination of nitrate and nitric acid by acid reduction and diazotization at elevated temperature

A rapid technique for determination of nitrate by acid reduction and diazotization at elevated temperature has been standardized. The technique is based on quantitative diazotization of sulfanilamide by nitrate on incubation in boiling water bath for 3, 5 or 10 min in presence of high concentration of HCl, ca. 64.5%. The diazotized sulfanilamide is coupled at room temperature to N-1-(naphthyl)-ethylenediamine dihydrochloride, and the chromophore evaluated spectrophotometrically at 540 nm. The technique provides linear estimate of nitrate over the test range of 0.5 through 10 μg N mL⁻¹ sample with all test incubation time periods using alkali nitrate and nitric acid as sources of nitrate anion. Urea treatment enables selective determination of nitrate in presence of nitrite with overall 99 ± 1% recovery, and without affecting nitrate determination (P > 0.1) or its regression coefficient. The technique has obvious advantages over metal-reduction technique. It is simple, rapid, selective in presence of nitrite, and an inexpensive method for routine determination of nitrate with detection range 0.5–10 μg N mL⁻¹ sample. Besides, the technique provides opportunity to detect nitric acid as low as 35 μM even in presence of other acids.     

Application of manganese(IV) dioxide microcolumn for determination and speciation of nitrite and nitrate using a flow injection analysis-flame atomic absorption spectrometry system

A flow injection (FI) method with flame atomic absorption spectrometry (FAAS) detection was developed for the determination and speciation of nitrite and nitrate in foodstuffs and wastewaters. The method is based on the oxidation of nitrite to nitrate using a manganese(IV) dioxide oxidant microcolumn where the flow of the sample through the microcolumn reduces the MnO₂ solid phase reagent to Mn(II), which is measured by FAAS. The absorbance of Mn(II) are proportional to the concentration of nitrite in the samples. The injected sample volume was 400 μL with a sampling rate of analyses was 90 h⁻¹ with a relative standard deviation better than 1.0% in a repeatability study. Nitrate is reduced to nitrite in proposed FI-FAAS system using a copperized cadmium microcolumn and analyzed as nitrite. The calibration curves were linear up to 20 mg L⁻¹ and 30 mg L⁻¹ with a detection limit of 0.07 mg L⁻¹ and 0.14 mg L⁻¹ for nitrite and nitrate, respectively. The results exhibit no interference from the presence of large amounts of ions. The method was successfully applied to the speciation of nitrite and nitrate in spiked natural water, wastewater and foodstuff samples. The precision and accuracy of the proposed method were comparable to those of the reference spectrophotometric method.     

Simultaneous determination of nitrite and nitrate in dew, rain, snow and lake water samples by ion-pair high-performance liquid chromatography

A simple, fast, sensitive and accurate reversed-phase ion-pair HPLC method for simultaneous determination of nitrite and nitrate in atmospheric liquids and lake waters has been developed. Separations were accomplished in less than 10 min using a reversed-phase C₁₈ column (150 mm × 2.00 mm i.d., 5 μm particle size) with a mobile phase containing 83% 3.0 mM ion-interaction reagent tetrabutylammonium hydroxide (TBA-OH) and 2.0 mM sodium phosphate buffer at pH 3.9 and 17% acetonitrile (flow rate, 0.4 mL/min). UV light absorption responses at 205 nm were linear over a wide concentration range from 100 μg/mL to the detection limits of 10 μg/L for nitrite and 5 μg/L nitrate. Quantitation was carried out by the peak area method. The relative standard deviation for the analysis of nitrite and nitrate was less than 3.0%. This method was applied for the simultaneous determination of nitrite and nitrate in dew, rain, snow and lake water samples collected in southeast Massachusetts. Nitrate was found being present at 4.79-5.99 μg/mL in dew, 1.20-2.63 μg/mL in rain, 0.32-0.60 μg/mL in snow and 0.12-0.23 μg/mL in lake water. Nitrite was only a minor species in dew (0.62-0.83 μg/mL), rain (<0.005-0.14 μg/mL), snow (0.021-0.032 μg/mL) and lake water (0.12-0.16 μg/mL). High levels of nitrite and nitrate observed in dew water droplets may constitute an important source of hydroxyl radicals in the sunny early morning.     

A simple and accurate method to determine nitrite and nitrate in serum based on high‐performance liquid chromatography with fluorescence detection

A simple method for accurate determination of nitrite and nitrate in serum was proposed to avoid the variation of nitrate reduction. For nitrite determination, serum samples were directly precipitated with methanol pre‐nitrate conversion, and then the supernatant reacted with 2,3‐diaminonaphthalene (DAN) to form 2,3‐naphthotriazole (NAT), which was quantitatively analyzed by high‐performance liquid chromatography coupled with fluorescence detection (HPLC‐FL). For nitrate determination, samples were firstly heated at 70°C for 10 min to inactivate endogenous reductase‐inhibiting proteins, then nitrate in the samples was quantitatively reduced to nitrite by reductase added experimentally. The difference in total nitrite concentrations between pre‐ and post‐nitrate conversion was used to calculate the amount of nitrate in the samples. In addition to good specificity, high sensitivity, satisfactory accuracy and reproducibility, our method is simple and suitable for the quantitative determination of nanomolar level of nitrite and nitrate in a large number of serum samples. Copyright © 2013 John Wiley & Sons, Ltd.     

A Novel Flow-Injection System for Simultaneous Determination of Nitrate and Nitrite Based on the Use of a Zinc Reductor and a Bulk Acoustic Wave Impedance Detector

A novel flow-injection system has been developed for the simultaneous determination of nitrate and nitrite present in water, foodstuffs, and human saliva. The system is based on the use of a zinc-filled reduction column and a bulk acoustic wave impedance sensor (BAWIS) as detector. With water as carrier stream, both nitrate and nitrite are converted on-line to ammonia, whereas with sulfamic acid, only nitrate is converted to ammonia. The ammonia formed diffuses across a PTFE membrane and is trapped in an acid stream causing a change in the solution conductance, which is monitored by a BAWIS detector. At a throughput of about 60 h⁻¹, the proposed system exhibited a linear response to the concentration of nitrate and nitrite from 2.5 μM to 1.00 mM, with detection limits of 1.7 and 1.8 μM, respectively, and the relative standard deviation of the peak heights (n= 6) ranged between 0.83 and 1.75% for the entire working range. In analysis of real samples, the simultaneous determination of nitrate and nitrite was achieved by the proposed method with a simple change of the carrier stream between water and sulfamic acid, and the results agreed well with those of conventional colorimetry.     

Multiparametric automated system for sulfate,nitrite and nitrate monitoring in drinking water and wastewater based on sequential injection analysis

An automated monitoring system for sulfate, nitrite and nitrate based on sequential injection analysis (SIA) was developed. For nitrite determination the modified Griess-Ilosvay method was used, whereas nitrate was previously reduced to nitrite using a cadmium column followed by nitrite determination. A turbidimetric method was carried out in order to determine sulfate. The results showed that the proposed SIA monitoring system constitutes an effective approach for nitrite, nitrate and sulfate determination since it is able to determine levels required by international agencies that regulate these parameters in water. Detection limits of 0.0207 mg N L^? 1, 0.0022 mg N L^? 1 and 3 mg SO₄^2? L^? 1 were obtained for nitrate, nitrite and sulfate, respectively. The developed method offers also typical characteristics of the multicommutated systems, as portability, low reagents consumption and the subsequently minimization of waste generation. The proposed system was successfully applied to drinking water and wastewater samples and validated with a certified river water sample (ION-96.3, LGC Standards).     

A simple miniaturised photometrical method for rapid determination of nitrate and nitrite in freshwater

A rapid, simple miniaturised photometrical method was developed for the determination of nitrate and/or nitrite in freshwater samples. All procedures, including sample buffering, reduction by copperised cadmium granules, colour development and absorbance determination, were completed in a 96-well microplate. The factors governing the nitrate reduction and its recovery were investigated in detail, and the optimised analysing conditions were established. Nitrate was quantitatively reduced by copperised cadmium granules with a high reduction efficiency (96.59 ± 0.96%). The proposed method gave a linear calibration ranging from 0.01 to 1.50 mg L⁻¹ for NO₂⁻-N and 0.02 to 1.50 mg L⁻¹ for NO₃⁻-N. The detection limits for nitrite and nitrate were 2 and 4 μg L⁻¹, respectively. The proposed method allowed at least 48 samples to be simultaneously analysed in duplicate, with good precision, within 90 min for nitrate and 30 min for nitrite, and was successfully applied to actual freshwater sample analysis with a recovery of 98.02 ± 1.04% for nitrite and 99.72 ± 1.39% for nitrate. This method produced accurate results comparable to standard methods, provided a much higher sample throughput than conventional methods and could be routinely used in actual freshwater sample monitoring.     

Simultaneous determination of nitrite and nitrate in human plasma by on-capillary preconcentration with field-amplified sample stacking

A simple method for the determination of nitrite and nitrate in human plasma has been developed using CZE with minimal sample preparation. Field‐amplified sample stacking (FASS) was used to achieve submicromolar detection by dilution of the plasma sample with deionized water. In CZE, the separation of nitrite and nitrate was achieved within 10 min without adding EOF modifier. The optimal condition was achieved with 50 mM phosphate buffer at pH 9.3. The ninefold diluted plasma samples were injected hydrodynamically for 40 s into a 60 cm×75 μm id uncoated fused‐silica capillary. The separation voltage was 20 kV (negative potential) and UV detection was performed at 214 nm. The linearity curves for nitrite and nitrate were obtained by the standard addition method. The estimated LODs for nitrite and nitrate in ninefold diluted plasma sample were 0.05 and 0.07 μM, respectively. The LODs for nitrite and nitrate in original plasma samples were 0.45 and 0.63 μM. The intra‐ and inter‐day precisions for both analytes were <2.6% and the recovery ranged between 92.3 and 113.3%. It was found that nitrite was more stable than nitrate in the plasma after the sample preparation. This proposed method was applied to a number of human plasma samples and the measured nitrite and nitrate concentrations in human plasma were consistent with the literature ranges.     

A method for the determination of the uranium content in U-Al alloys

The paper describes a method for the determination of uranium content in enriched U–Al alloys. The mass-spectrometric isotope dilution method was proposed and verified for the determination of uranium in these materials. A solution of natural uranylnitrate, prepared by dissolution of standard reference material NBS-U-960, was used as a spike. Uranium was separated from aluminium in the form of chloro complexes by sorption on anion exchange resin Dowex 1-X8 from 9M HCl. The error of determination lies in an interval 0.03–0.07% rel. for uranium contents 18.2–22.8 wt%.     

Rapid simultaneous determination of nitrate and nitrite on a centrifugal microfluidic device

A centrifugal microfluidic device was developed for the rapid sequential determination of two critical environmental species, nitrate and nitrite, in water samples. The nitrate is reduced to nitrite and the nitrite is derivatized. The analytes are determined spectrophotometrically through the disc with a 1.4 mm pathlength. The detection limits are 0.05 and 0.16 mg L⁻¹ for nitrite and nitrate respectively. The use of powdered reagents, the 100 μL sample required and the design of the device suggest that it would be suitable for field use.     

A novel spectrofluorimetric method for the ultra trace analysis of nitrite and nitrate in aqueous medium and its application to air, water, soil and forensic samples

A highly sensitive and virtually specific method has been developed for the trace and ultra trace 5 ng ml⁻¹-1 μg ml⁻¹ fluorimetric analysis of nitrite. The method is based on the quenching action of nitrite on the native fluorescence of murexide (ammonium purpurate) [λ_ex=349.0 nm, λ_em=444.5 nm] in the acid range of 0.045-0.315 (M) H₂SO₄. The method is very precise and accurate (S.D.=±0.4877 and R.S.D.=0.4878% for the determination of 0.1 μg ml⁻¹ of nitrite in 11 replicates). Relatively large excesses of over 35 cations and anions do not interfere. The proposed technique has been successfully applied for the determination of nitrite and nitrate in ground water, surface water and sea water, nitrite in soil and nitrate in forensic samples. The method has also been extended for the analysis of NO_x in air.     

Determination of uranium in seawater by flow-injection preconcentration on dodecylamidoxime-impregnated resin and spectrophotometric detection

A flow injection method has been developed for the determination of uranium in seawater combining the on-line preconcentration with spectrophotometric detection. An aliquot (10 mL) of the seawater sample adjusted to pH 5.5 was injected into the analytical system and uranium was adsorbed on the column packed with styrene-divinylbenzene copolymer resin (Bio-Beads SM-2) modified with dodecylamidoxime which showed high selectivity to uranium. Uranium was then eluted with 0.01 M hydrochloric acid and detected spectrophotometrically after the reaction with Chlorophosphonazo III. Interference from calcium and strontium was masked with cyclohexanediaminetetraacetic acid added to the chromogenic reagent solution. The sample throughput, the detection limit (3σ), and the preconcentration factor were 23 per hour, 0.13 μg/L, and 20, respectively, when the sample injection volume was kept at 10 mL. The precision at the 2 μg/L level was less than 4% (RSD). The proposed method was applied to the determination of uranium in the seawater samples collected off the Boso peninsula, Japan and the uranium concentration was found to be ca. 3 μg/L, which is close to the literature data. The yield of the recovery test ranged from 95% to 99%.     

Development of a simple method for the determination of nitrite and nitrate in groundwater by high-resolution continuum source electrothermal molecular absorption spectrometry

In this work, it was developed a method for the determination of nitrite and nitrate in groundwater by high-resolution continuum source electrothermal molecular absorption spectrometry of NO produced by thermal decomposition of nitrate in a graphite furnace. The NO line at 215.360 nm was used for all analytical measurements and the signal obtained by integrated absorbance of three pixels. A volume of 20 μL of standard solution or groundwater sample was injected into graphite furnace and 5 μL of a 1% (m/v) Ca solution was co-injected as chemical modifier. The pyrolisis and vaporization temperatures established were of 150 and 1300 °C, respectively. Under these conditions, it was observed a difference of thermal stability among the two nitrogen species in the presence of hydrochloric acid co-injected. While that the nitrite signal was totally suppressed, nitrate signal remained nearly stable. This way, nitrogen can be quantified only as nitrate. The addition of hydrogen peroxide provided the oxidation of nitrite to nitrate, which allowed the total quantification of the species and nitrite obtained by difference. A volume of 5 μL of 0.3% (v/v) hydrochloric acid was co-injected for the elimination of nitrite, whereas that hydrogen peroxide in the concentration of 0.75% (v/v) was added to samples or standards for the oxidation of nitrite to nitrate. Analytical curve was established using standard solution of nitrate. The method described has limits of detection and quantification of 0.10 and 0.33 μg mL⁻¹ of nitrogen, respectively. The precision, estimated as relative standard deviation (RSD), was of 7.5 and 3.8% (n = 10) for groundwater samples containing nitrate–N concentrations of 1.9 and 15.2 μg mL⁻¹, respectively. The proposed method was applied to the analysis of 10 groundwater samples and the results were compared with those obtained by ion chromatography method. In all samples analyzed, the concentration of nitrite–N was always below of the limit of quantification of both the methods. The concentrations of nitrate–N varied from 0.58 to 15.5 μg mL⁻¹. No significant difference it was observed between the results obtained by both methods for nitrate–N, at the 95% confidence level.